Throttle Body Guide - Air Apparent

Understanding throttle bodies

Richard Holdener

July 29, 2011

How much power is a throttle body upgrade worth? The question seems simple enough, but the answer is somewhat less so. Power gains seem to range from a little as 0 hp to as much as 25-30 hp. It seems like a pretty broad range of power gains for a simple bolt-on.

In point of fact, we are here to tell you that a throttle-body swap netted over 60 hp on a Kenne Bell supercharged application. In reality, the question is not so much can a throttle body swap net a sizable jump in power, but rather, does such an upgrade always results in said gains. The quick and dirty answer is a resounding no, but the reason behind it is certainly worth a closer look.

In its simplest form, the throttle body is nothing more than an air valve. There is no magic to the workings of throttle body, though there is some magic to maximizing the flow rate through it. A given opening will flow a certain amount of air, but radiused entries, thin throttle blades, and the elimination of hardware in the air stream all combine to further increase the airflow of a given bore size. It stands to reason that a 90mm throttle body should outflow an 80mm throttle body, but it is possible for a well-designed 80mm throttle body to outflow a poorly designed 90mm.

Back in the early days of the 5.0L Mustang, we marveled at the ability to upgrade to a 65mm throttle body. Modern motors are equipped with 90mm and dual 75mm throttle bodies right from the factory. Contrary to popular belief, a larger throttle opening does not reduce low-speed power, but the effect on throttle response can be dramatic. The reason is that (compared to a smaller throttle body), any throttle position will offer a sizable increase in airflow. Opening a 90mm throttle body even 3-5 percent is like opening a smaller throttle body 10-12 percent. This makes part-throttle driving difficult as minor throttle angle changes result in dramatic power differences.

Now that we have established that a larger throttle body will outflow a smaller version (assuming equal design quality), we can take a look at why the installation of a larger throttle body may or may not improve power. An example works well here. According to testing performed at Kenne Bell, the stock '05-up Three-Valve dual 55mm throttle body flows 890 cfm. Using the formula that 1 hp requires 1.5 cfm, we see that this stock throttle body might support as much as 593 hp, or way more than the factory-rated 300 hp.

The first obstacle in terms of power production is the fact that the throttle body is not the only component in the induction system. The flow rate of the throttle body is only as good as the supporting components. In the case of the '05-up GT, the flow rate of the complete induction system is only 509 cfm, a significant drop from the 890 cfm offered by the throttle body alone.

According to our formula, the stock air intake system is capable of supporting 339 hp, or just slightly more than the stock power output. According to this data, power gains will come from the air intake system and not the throttle body on a stock or mildly modified motor. Of course, installation of a better exhaust and revised cams can increase efficiency and allow more power than those number would suggest.

Another important factor when it comes to the power gains offered by the throttle body is engine combination. From the most basic standpoint, the higher the power output of the test motor, the larger the throttle body required. Upgrading a throttle body already capable of supporting 600 hp with a larger version capable of supporting 700 hp on a 300hp motor will have predictable results.

The 600hp throttle body is already oversized for the application, so there is no need to upgrade. This is especially critical on supercharged applications, where elevated power levels are more commonplace. You'd be hard pressed to find many 600hp normally aspirated Three-Valve 4.6L motors, but add a supercharger to the mix and they are everywhere.